Silica-based sols

ABSTRACT

The invention relates to an aqueous silica-based sol comprising a nitrogen-containing organic compound and silica-based particles with a specific surface area of at least 300 square meters per gram of silica. The invention further relates to a process for the production of an aqueous silica-based sol comprising a nitrogen-containing organic compound which comprises incorporating a nitrogen-containing organic compound into a silica-based sol containing silica-based particles with a specific surface area of at least 300 square meters per gram of silica. The invention also relates to the use of an aqueous silica-based sol comprising a nitrogen-containing organic compound and silica-based particles with a specific surface area of at least 300 square meters per gram of silica as a flocculating agent in the production of pulp and paper and in water purification. The invention further relates to a process for the production of paper from an aqueous suspension containing cellulosic fibers, and optional fillers, which comprises adding to the suspension (i) an aqueous silica-based sol comprising an organic nitrogen-containing compound and (ii) at least one charged organic polymer, forming and draining the suspension on a wire.

[0001] The present invention generally relates to silica-based solscomprising an organic nitrogen-containing compound which sols aresuitable for use as drainage and retention aids in papermaking. Moreparticularly, the invention relates to silica-based sols, a process forthe production of silica-based sols, and a process for the production ofpaper in which silica-based sols are used as additives.

BACKGROUND

[0002] In the papermaking art, an aqueous suspension containingcellulosic fibres, and optional fillers and additives, referred to asstock, is fed into a headbox which ejects the stock onto a forming wire.Water is drained from the stock through the forming wire so that a wetweb of paper is formed on the wire, and the paper web is furtherdewatered and dried in the drying section of the paper machine. Drainageand retention aids are conventionally introduced into the stock in orderto facilitate drainage and to increase adsorption of fine particles ontothe cellulosic fibres so that they are retained with the fibres on thewire.

[0003] Silica-based particles are widely used as drainage and retentionaids in combination with charged organic polymers like anionic andcationic acrylamide-based polymers and cationic and amphoteric starches.Such additive systems are disclosed in U.S. Pat. Nos. 4,388,150;4,961,825; 4,980,025; 5,368,833; 5,603,805; 5,607,552; 5,858,174; and6,103,064; and International Patent Applications WO 00/66491 and WO00/66492. These systems are among the most efficient drainage andretention aids now in use.

[0004] Silica-based particles suitable for use as drainage and retentionaids are normally supplied in the form of aqueous colloidal dispersions,so-called sols. Such commercially used silica-based sols usually have asilica content of about 7-15% by weight and contain particles with aspecific surface area of at least 300 m²/g. Sols of silica-basedparticles with higher specific surface areas are usually more dilute toimprove storage stability and avoid gel formation.

[0005] It would be advantageous to be able to provide silica-based solswith further improved drainage and retention performance and even betterstability. It would also be advantageous to be able to provide a processfor preparing silica-based sols exhibiting improved drainage, retentionand stability properties. It would also be advantageous to be able toprovide a papermaking process with improved drainage and retention.

THE INVENTION

[0006] In accordance with the present invention there is providedamine-modified silica-based sols which are suitable for use as drainageand retention aids in papermaking. The term “drainage and retentionaids”, as used herein, refers to one or more components (aids, agents oradditives) which, when added to a papermaking stock, give betterdrainage and/or retention than is obtained when not adding the said oneor more components. The amine-modified silica-based sols of theinvention result in improved drainage and/or retention when used inconjunction with charged organic polymers. Hereby the present inventionmakes it possible to increase the speed of the paper machine and to usea lower dosage of additives to give a corresponding drainage and/orretention effect, thereby leading to an improved papermaking process andeconomic benefits. The silica-based sols of the invention furtherexhibit very good stability over extended periods of time, notably veryhigh surface area stability and high stability towards gelation, andhence they can be prepared and shipped at high specific surface areasand high silica concentrations. The sols have improved capability tomaintain the high specific surface area on storage at high silicaconcentrations.

[0007] The present invention thus relates to silica-based solscomprising an organic nitrogen-containing compound, the preparation ofsuch sols and their use, as further defined in the appended claims. Theinvention also relates to a process for the production of paper from anaqueous suspension containing cellulosic fibres, and optional fillers,which comprises adding to the suspension a silica-based sol comprisingan organic nitrogen-containing compound and at least one charged organicpolymer, forming and draining the suspension on a wire, as furtherdefined in the appended claims.

[0008] The silica-based sols according to the invention are aqueous solsthat contain anionic silica-based particles, i.e. particles based onSiO₂ or silicic acid, including colloidal silica, different types ofpolysilicic acid, polysilicate microgels, colloidal borosilicates,aluminium-modified silica or aluminium silicates, polyaluminosilicatemicrogels, and mixtures thereof. The particles are preferably colloidal,i.e. in the colloidal range of particle size, and preferably amorphousor essentially amorphous. The silica-based particles suitably have anaverage particle size below about 50 nm, preferably below about 20 nmand more preferably in the range of from about 1 to about 10 nm. Asconventional in silica chemistry, particle size refers to the averagesize of the primary particles, which may be aggregated ornon-aggregated.

[0009] The silica-based sols according to the invention contains anorganic nitrogen-containing compound, for example an amine which can beselected from primary amines, secondary amines, tertiary amines andquaternary amines, the latter also referred to as quaternary ammoniumcompounds. The amines can be aromatic, i.e. containing one or morearomatic groups, or aliphatic; the aliphatic amines usually beingpreferred. The nitrogen-containing compound is preferably water-solubleor water-dispersible. The amine can be uncharged or cationic. Examplesof cationic amines include acid addition salts of primary, secondary andtertiary amines and, preferably, quaternary ammonium compounds, as wellas their hydroxides. The organic nitrogen-containing compound usuallyhas a molecular weight below 1,000, suitably below 500 and preferablybelow 300. Preferably, a low molecular weight organicnitrogen-containing compound is used, for example those compounds havingup to 25 carbon atoms, suitably up to 20 carbon atoms, preferably from 2to 12 carbon atoms and most preferably from 2 to 8 carbon atoms. In apreferred embodiment, the organic nitrogen-containing compound has oneor more oxygen-containing substituents, for example with oxygen in theform of hydroxyl groups and/or alkyloxy groups. Examples of preferredsubstituents of this type include hydroxy alkyl groups, e.g. ethanolgroups, and methoxy and ethoxy groups. The organic nitrogen-containingcompounds may include one or more nitrogen atoms, preferably one or two.Preferred amines include those having a pKa value of at least 6,suitably at least 7 and preferably at least 7.5.

[0010] Examples of suitable primary amines, i.e. amines having oneorganic substituent, include alkylamines, e.g. propylamine, butylamine,cyclohexylamine, alkanolamines, e.g. ethanolamine, andalkoxyalkylamines, e.g. 2-methoxyethylamine. Examples of suitablesecondary amines, i.e. amines having two organic substituents, includedialkylamines, e.g. diethylamine, dipropylamine and di-isopropylamine,dialkanolamines, e.g. diethanolamine, and pyrrolidine. Examples ofsuitable tertiary amines, i.e. amines having three organic substituents,include trialkylamines, e.g. triethylamine, trialkanolamines, e.g.triethanolamine, N,N-dialkylalkanolamines, e.g.N,N-dimethylethanolamine. Examples of suitable quaternary amines, orquaternary ammonium compounds, i.e. amines having four organicsubstituents, include tetraalkanolamines, e.g. tetraethanol ammoniumhydroxide and tetraethanol ammonium chloride, quaternary amines orammonium compounds with both alkanol and alkyl substituents such asN-alkyltrialkanolamines, e.g. methyltriethanolammonium hydroxide andmethyltriethanolammonium chloride, N,N-dialkyldialkanolamines, e.g.dimethyldiethanolammonium hydroxide and dimethyldiethanolammoniumchloride, N,N,N-trialkylalkanolamines, e.g. choline hydroxide andcholine chloride, N,N,N-trialkylbenzylamines, e.g.dimethylcocobenzylammonium hydroxide and dimethylcocobenzylammoniumchloride, tetraalkylammonium salts, e.g. tetramethylammonium hydroxide,tetramethylammonium chloride, tetraethylammonium hydroxide,tetraethylammonium chloride, tetrapropylammonium hydroxide,tetrapropylammonium chloride, diethyldimethylammonium hydroxide,diethyldimethylammonium chloride, triethylmethylammonium hydroxide andtriethylmethylammonium chloride. Examples of suitable diamines includeaminoalkylalkanolamines, e.g. aminoethylethanolamine, piperazine andnitrogen-substituted piperazines having one or two lower alkyl groups of1 to 4 carbon atoms. Examples of preferred organic nitrogen-containingcompounds include triethanolamine, diethanolamine, dipropylamine,aminoethylethanolamine, 2-methoxyethylamine, N,N-dimethylethanolamine,choline hydroxide, choline chloride, tetramethylammonium hydroxide,tetraethylammonium hydroxide and tetraethanol ammonium hydroxide.

[0011] The molar ratio of SiO₂ to N of the silica-based sols is usuallyfrom 1:1 to 50:1, suitably from 2:1 to 40:1 and preferably from 2.5:1 to25:1.

[0012] The specific surface area of the amine-modified aqueoussilica-based sols of the invention is suitably at least 90 m²/g aqueoussol, i.e. based on the weight of aqueous sol, preferably at least 100m²/g aqueous sol, more preferably at least 115 m²/g aqueous sol and mostpreferably at least 150 m²/g aqueous sol. Generally, the specificsurface area of the aqueous sol obtained can be up to about 300 m²/gaqueous sol, suitably up to 250 m²/g aqueous sol, preferably up to 240m²/g aqueous sol.

[0013] The specific surface area of the silica-based particles issuitably at least 300 m²/g SiO₂, i.e. based on the weight of SiO₂,preferably at least 400 m²/g SiO₂ and most preferably at least 550 m²/gSiO₂, notably at least 700 m²/g SiO₂. Generally, the specific surfacearea of the particles can be up to about 1700 m²/g SiO₂. In a preferredembodiment of the invention, the specific surface area of thesilica-based particles is up to about 1000 m²/g SiO₂, suitably fromabout 550 to 950 m²/g SiO₂. In another preferred embodiment of theinvention, the specific surface area of the silica-based particles isfrom about 1000 to 1700 m²/g SiO₂, suitably from 1050 to 1500 m²/g SiO₂.

[0014] The specific surface area can be measured by means of titrationwith NaOH in known manner, e.g. as described by Sears in AnalyticalChemistry 28(1956):12, 1981-1983 and in U.S. Pat. No. 5,176,891, afterappropriate removal of or adjustment for the organic nitrogen-containingcompound and any other compounds present in the sample that may disturbthe titration like aluminium and boron species. When expressed in squaremeters per gram of aqueous sol, the specific surface area represents thespecific surface area that is available per gram of aqueous silica-basedsol. When expressed in square meters per gram of silica, the specificsurface area represents the average specific surface area of thesilica-based particles present in the sol.

[0015] The silica-based sols usually have an S-value within the range offrom 10 to 60%, suitably from 15 to 50%, preferably from 15 to 40% andmost preferably from 20 to 35%. The S-value can be measured andcalculated as described by Iler & Dalton in J. Phys. Chem. 60(1956),955-957. The S-value, which is affected by silica concentration, densityand viscosity of the silica-based sol, can be seen as an indication ofthe degree of particle aggregation or interparticle attraction and alower S-value indicates a higher degree aggregation.

[0016] The silica-based sols should suitably have a silica content of atleast 3% by weight but it is more suitable that the silica content iswithin the range of from 10 to 60% by weight and preferably from 12 to40% by weight. In order to reduce storage facilities and to simplifyshipping and reduce transportation costs, it is generally preferable toship high concentration silica-based sols but it is of course possibleand usually preferable to dilute the sols to substantially lower silicacontents prior to use, for example to silica contents within the rangeof from 0.05 to 5% by weight, in order to improve mixing with thefurnish components.

[0017] The viscosity of the silica-based sols can vary depending on, forexample, the silica content of the sol, specific surface area of thesilica-based particles and the organic nitrogen-containing compoundused. Usually, the viscosity is at least 1.5 cP, normally within therange of from 2 to 100 cP, suitably from 2 to 70 cP and preferably from2.5 to 40 cP. The viscosity, which is suitably measured on sols having asilica content of at least 10% by weight, can be measured by means ofknown technique, for example using a Brookfield LVDV II+ viscosimeter.The pH of the silica-based sols according to the invention is usuallyfrom 7 to 14, suitably from 8 to 13 and preferably from 9 to 12.

[0018] The silica-based sols of this invention are preferably stable.The term “stable silica-based sol”, as used herein, refers tosilica-based sols which when subjected to storage or ageing for onemonth at 20° C. in dark and non-agitated conditions exhibit an increasein viscosity of less than 100 cP. Suitably the viscosity increase, ifany, is less than 50 cP and preferably less than 30 cP when the sols aresubjected to the above conditions.

[0019] In addition to the nitrogen-containing compound, the silica-basedsols according to the invention may also contain other elements, forexample aluminium and boron. Such elements may be present as a result ofmodification using aluminium-containing and boron-containing compounds,respectively. If aluminium is used, the sols can have a molar ratio ofSiO₂ to Al₂O₃ within the range of from 4:1 to 1500:1, suitably from 8:1to 1000:1 and preferably from 15:1 to 500:1. If boron is used, the solscan have a molar ratio of SiO₂ to B within the range of from 4:1 to1500:1, suitably from 8:1 to 1000:1 and preferably from 15:1 to 500:1.

[0020] The aqueous silica-based sols according to the invention can beproduced by incorporating a nitrogen-containing compound, for exampleany of the ones described above and having the above characteristics,into a silica-based sol, optionally followed by concentration of thesilica-based sol. The silica-based sol to be used suitably containsanionic silica-based particles. Preferably the particles are colloidaland amorphous or essentially amorphous. The specific surface area of thesilica-based particles is suitably at least 300 m²/g SiO₂, i.e. based onthe weight of SiO₂, preferably at least 400 m²/g SiO₂ and mostpreferably at least 550 m²/g SiO₂, notably at least 700 m²/g SiO₂.Generally, the specific surface area of the particles can be up to about1700 m²/g SiO₂. In a preferred embodiment of the invention, the specificsurface area of the silica-based particles is up to about 1000 m²/gSiO₂, suitably from about 550 to 950 m²/g SiO₂. In another preferredembodiment of the invention, the specific surface area of thesilica-based particles is from about 1000 to 1700 m²/g SiO₂, suitablyfrom 1050 to 1500 m²/g SiO₂. The silica-based sol to be used in theprocess usually has an S-value within the range of from 10 to 60%,suitably from 15 to 50%, preferably from 15 to 40% and most preferablyfrom 20 to 35%.

[0021] The aqueous silica-based sol to be used in the process accordingto the invention usually has a pH within the range of from 1 to 11. Inone preferred aspect of this invention, the pH of the aqueoussilica-based sol to be used is within the range of from 1 to 4, usuallyan acid silica-based sol or polysilicic acid. In another preferredaspect of this invention, the pH of the aqueous silica-based sol to beused is within the range of from 4 to 11, suitably from 7 and mostpreferably from 8 up to 11.0, preferably up to 10.5.

[0022] Acid silica-based sols can be prepared starting from aconventional aqueous silicate solution like alkali water glass, e.g.potassium or sodium water glass, preferably sodium water glass. Themolar ratio of SiO₂ to M₂O, where M is alkali metal, e.g. sodium,potassium, ammonium, or a mixture thereof, in the silicate solution orwater glass is suitably within the range of from 1.5:1 to 4.5:1,preferably from 2.5:1 to 3.9:1, and pH is usually around 13 or above 13.Suitably a dilute silicate solution or water glass is used which canhave an SiO₂ content of from about 3 to about 12% by weight, preferablyfrom about 5 to about 10% by weight. The silicate solution or waterglass is normally acidified to a pH of from about 1 to about 4. Theacidification can be carried out in known manner by addition of mineralacids, e.g. sulphuric acid, hydrochloric acid and phosphoric acid, oroptionally with other chemicals known as suitable for acidification ofwater glass, e.g. ammonium sulphate and carbon dioxide. However, it ispreferred that the acidification is carried out by means of an acidcation exchanger which, among other things, lead to more stableproducts. The acidification is preferably carried out by means of astrongly acid cation exchange resin, for example of sulfonic acid type.It is preferred that the acidification is carried out to a pH of fromabout 2 to 4, most preferably from about 2.2 to 3.0. The obtained acidsol, or polysilicic acid, contains particles with a high specificsurface area, normally above 1000 m²/g SiO₂ and usually around about1300 to 1500 m²/g SiO₂. Acid silica-based sols can also be prepared byacidification of an alkaline silica-based sol, for example by means ofacidification as described above.

[0023] The organic nitrogen-containing compound is then incorporatedinto the acid sol, optionally in combination with alkali, e.g. lithiumhydroxide, sodium hydroxide, potassium hydroxide and ammonium hydroxide,or an aqueous silicate solution as defined above. The organicnitrogen-containing compound and the alkali can be added simultaneously,either separately or in admixture, or sequentially, e.g. addition ofnitrogen-containing compound followed by addition of alkali. The amountof organic nitrogen-containing compound is usually such that theabove-mentioned molar ratio of SiO₂ to nitrogen (N) is obtained. The pHof the organic nitrogen-containing compound modified silica-based sol isusually from 7 to 13, suitably from 8 to 12.5 and preferably from 9 to12.

[0024] In a preferred embodiment of the invention, the silica-based solobtained after incorporation of the organic nitrogen-containing compoundis further concentrated. Concentration can be carried out in knownmanner such as, for example, by osmotic methods, evaporation andultrafiltration. The concentration is suitably carried out to producesilica contents of at least 10% by weight, preferably from 10 to 60% byweight, and more preferably from 12 to 40% by weight. The concentrationis further usually carried out so that the silica-based sol obtained inthe process has a specific surface area of at least 90 m²/g aqueous sol,i.e., based on the weight of aqueous sol, suitably at least 100 m²/gaqueous sol, preferably at least 115 m²/g aqueous sol and mostpreferably at least 150 m²/g aqueous sol. Generally, the specificsurface area of the aqueous sol obtained can be up to about 300 m²/gaqueous sol, suitably up to 250 m²/g aqueous sol, preferably up to 240m²/g aqueous sol.

[0025] According to the present process, silica-based sols, notablystable silica-based sols, having the above characteristics can beprepared and the produced sols exhibit good storage stability and can bestored for several months without any substantial decrease of thespecific surface area and without gelation.

[0026] The organic nitrogen-containing compound modified silica-basedsols of this invention are suitable for use as drainage and retentionaids in papermaking. The silica-based sols can be used in combinationwith organic polymers which can be selected from anionic, amphoteric,non-ionic and cationic polymers and mixtures thereof, herein alsoreferred to as “main polymer”. The use of such polymers as drainage andretention aids is well known in the art. The polymers can be derivedfrom natural or synthetic sources, and they can be linear, branched orcross-linked. Examples of generally suitable main polymers includeanionic, amphoteric and cationic starches, anionic, amphoteric andcationic guar gums, and anionic, amphoteric and cationicacrylamide-based polymers, as well as cationic poly(diallyldimethylammonium chloride), cationic polyethylene imines, cationic polyamines,polyamidoamines and vinylamide-based polymers, melamine-formaldehyde andurea-formaldehyde resins. Suitably the silica-based sols are used incombination with at least one cationic or amphoteric polymer, preferablycationic polymer. Cationic starch and cationic polyacrylamide areparticularly preferred polymers and they can be used singly, togetherwith each other or together with other polymers, e.g. other cationicpolymers or anionic polyacrylamide. The molecular weight of the mainpolymer is suitably above 1,000,000 and preferably above 2,000,000. Theupper limit is not critical; it can be about 50,000,000, usually30,000,000 and suitably about 25,000,000. However, the molecular weightof polymers derived from natural sources may be higher.

[0027] When using the silica-based sols in combination with mainpolymer(s) as mentioned above, it is further preferred to use at leastone low molecular weight (hereinafter LMW) cationic organic polymer,commonly referred to and used as anionic trash catchers (ATC). ATCs areknown in the art as neutralizing and/or fixing agents for detrimentalanionic substances present in the stock and the use thereof incombination with drainage and retention aids often provide furtherimprovements in drainage and/or retention. The LMW cationic organicpolymer can be derived from natural or synthetic sources, and preferablyit is an LMW synthetic polymer. Suitable organic polymers of this typeinclude LMW highly charged cationic organic polymers such as polyamines,polyamideamines, polyethyleneimines, homo- and copolymers based ondiallyldimethyl ammonium chloride, (meth)acrylamides and(meth)acrylates. In relation to the molecular weight of the mainpolymer, the molecular weight of the LMW cationic organic polymer ispreferably lower; it is suitably at least 1,000 and preferably at least10,000. The upper limit of the molecular weight is usually about700,000, suitably about 500,000 and usually about 200,000. Preferredcombinations of polymers that can be co-used with the silica-based solsof this invention include LMW cationic organic polymer in combinationwith main polymer(s), such as, for example, cationic starch and/orcationic polyacrylamide, anionic polyacrylamide as well as cationicstarch and/or cationic polyacrylamide in combination with anionicpolyacrylamide.

[0028] The components of the drainage and retention aids according tothe invention can be added to the stock in conventional manner and inany order. When using drainage and retention aids comprising asilica-based sol and an organic polymer, e.g. a main polymer, it ispreferred to add the polymer to the stock before adding the silica-basedsol, even if the opposite order of addition may be used. It is furtherpreferred to add the main polymer before a shear stage, which can beselected from pumping, mixing, cleaning, etc., and to add thesilica-based sol after that shear stage. LMW cationic organic polymers,when used, are preferably introduced into the stock prior to introducingthe main polymer. Alternatively, the LMW cationic organic polymer andthe main polymer can be introduced into stock essentiallysimultaneously, either separately or in admixture, for example asdisclosed in U.S. Pat. No. 5,858,174, which is hereby incorporatedherein by reference. The LMW cationic organic polymer and the mainpolymer are preferably introduced into the stock prior to introducingthe silica-based sol.

[0029] In a preferred embodiment of this invention, the silica-basedsols are used as drainage and retention aids in combination with atleast one organic polymer, as described above, and at least onealuminium compound. Aluminium compounds can be used to further improvethe drainage and/or retention performance of stock additives comprisingsilica-based sols. Suitable aluminium salts include alum, aluminates,aluminium chloride, aluminium nitrate and polyaluminium compounds, suchas polyaluminium chlorides, polyaluminium sulphates, polyaluminiumcompounds containing both chloride and sulphate ions, polyaluminiumsilicate-sulphates, and mixtures thereof. The polyaluminium compoundsmay also contain other anions, for example anions from phosphoric acid,organic acids such as citric acid and oxalic acid. Preferred aluminiumsalts include sodium aluminate, alum and polyaluminium compounds. Thealuminium compound can be added before or after the addition of thesilica-based sol. Alternatively, or additionally, the aluminium compoundcan be added simultaneously with the silica-based sol at essentially thesame point, either separately or in admixture with it, for example asdisclosed by U.S. Pat. No. 5,846,384 which is hereby incorporated hereinby reference. In many cases, it is often suitable to add an aluminiumcompound to the stock early in the process, for example prior to theother additives.

[0030] The components of the drainage and retention aids according tothe invention are added to the stock to be dewatered in amounts whichcan vary within wide limits depending on, inter alia, type and number ofcomponents, type of furnish, filler content, type of filler, point ofaddition, etc. Generally the components are added in an amount that givebetter drainage and/or retention than is obtained when not adding thecomponents. The silica-based sol is usually added in an amount of atleast 0.001% by weight, often at least 0.005% by weight, calculated asSiO₂ and based on dry stock substance, i.e. cellulosic fibres andoptional fillers, and the upper limit is usually 1.0% and suitably 0.5%by weight. The main polymer is usually added in an amount of at least0.001%, often at least 0.005% by weight, based on dry stock substance,and the upper limit is usually 3% and suitably 1.5% by weight. Whenusing an LMW cationic organic polymer in the process, it can be added inan amount of at least 0.05%, based on dry substance of the stock to bedewatered. Suitably, the amount is in the range of from 0.07 to 0.5%,preferably in the range from 0.1 to 0.35%. When using an aluminiumcompound in the process, the total amount introduced into the stock tobe dewatered depends on the type of aluminium compound used and on othereffects desired from it. It is for instance well known in the art toutilise aluminium compounds as precipitants for rosin-based sizingagents. The total amount added is usually at least 0.05%, calculated asAl₂O₃ and based on dry stock substance. Suitably the amount is in therange of from 0.1 to 3.0%, preferably in the range from 0.5 to 2.0%.

[0031] In a preferred embodiment of the invention, the process is usedin the manufacture of paper from a suspension containing cellulosicfibers, and optional fillers, having a high conductivity. Usually, theconductivity of the stock that is dewatered on the wire is at least 0.75mS/cm, suitably at least 2.0 mS/cm, preferably at least 3.5 mS/cm. Verygood drainage and retention results have been observed at conductivitylevels of at least 5.0 mS/cm. Conductivity can be measured by standardequipment such as, for example a WTW LF 539 instrument supplied byChristian Berner. The values referred to above are suitably determinedby measuring the conductivity of the cellulosic suspension that is fedinto or present in the headbox of the paper machine or, alternatively,by measuring the conductivity of white water obtained by dewatering thesuspension. High conductivity levels mean high contents of salts(electrolytes) which can be derived from the cellulosic fibres andfillers used to form the stock, in particular in integrated mills wherea concentrated aqueous fibre suspension from the pulp mill normally ismixed with water to form a dilute suspension suitable for papermanufacture in the paper mill. The salt may also be derived from variousadditives introduced into the stock, from the fresh water supplied tothe process, or be added deliberately, etc. Further, the content ofsalts is usually higher in processes where white water is extensivelyrecirculated, which may lead to considerable accumulation of salts inthe water circulating in the process.

[0032] The present invention further encompasses papermaking processeswhere white water is extensively recirculated (recycled), i.e. with ahigh degree of white water closure, for example where from 0 to 30 tonsof fresh water are used per ton of dry paper produced, usually less than20, suitably less than 15, preferably less than 10 and notably less than5 tons of fresh water per ton of paper. Recirculation of white waterobtained in the process suitably comprises mixing the white water withcellulosic fibres and/or optional fillers to form a suspension to bedewatered; preferably it comprises mixing the white water with asuspension containing cellulosic fibres, and optional fillers, beforethe suspension enters the forming wire for dewatering. The white watercan be mixed with the suspension before, between simultaneous with orafter introducing the drainage and retention aids. Fresh water can beintroduced in the process at any stage; for example, it can be mixedwith cellulosic fibres in order to form a suspension, and it can bemixed with a suspension containing cellulosic fibres to dilute it so asto form the suspension to be dewatered, before or after mixing the stockwith white water and before, between, simultaneous with or afterintroducing the components of drainage and retention aids.

[0033] Further additives which are conventional in papermaking can ofcourse be used in combination with the additives according to theinvention, such as, for example, dry strength agents, wet strengthagents, optical brightening agents, dyes, sizing agents like rosin-basedsizing agents and cellulose-reactive sizing agents, e.g. alkyl andalkenyl ketene dimers and ketene multimers, alkyl and alkenyl succinicanhydrides, etc. The cellulosic suspension, or stock, can also containmineral fillers of conventional types such as, for example, kaolin,china clay, titanium dioxide, gypsum, talc and natural and syntheticcalcium carbonates such as chalk, ground marble and precipitated calciumcarbonate.

[0034] The process of this invention is used for the production ofpaper. The term “paper”, as used herein, of course include not onlypaper and the production thereof, but also other cellulosicfibre-containing sheet or web-like products, such as for example boardand paperboard, and the production thereof. The process can be used inthe production of paper from different types of suspensions ofcellulose-containing fibres and the suspensions should suitably containat least 25% by weight and preferably at least 50% by weight of suchfibres, based on dry substance. The suspension can be based on fibresfrom chemical pulp such as sulphate, sulphite and organosolv pulps,mechanical pulp such as thermomechanical pulp, chemo-thermomechanicalpulp, refiner pulp and groundwood pulp, from both hardwood and softwood,and can also be based on recycled fibres, optionally from de-inkedpulps, and mixtures thereof. The pH of the suspension, the stock, can bewithin the range of from about 3 to about 10. The pH is suitably above3.5 and preferably within the range of from 4 to 9.

[0035] The invention is further illustrated in the following Exampleswhich, however, are not intended to limit the same. Parts and % relateto parts by weight and % by weight, respectively, and all solutions areaqueous, unless otherwise stated.

EXAMPLE 1

[0036] A sodium hydroxide stabilized silica sol containing silicaparticles with a specific surface area of around 800 m²/g SiO₂ wasdeionized with a cationic ion-exchange resin saturated with hydrogenions. The resulting acid silica sol had a pH of 2.6, SiO₂ content of9.15% by weight and contained silica particles with a specific surfacearea of 820 m²/g SiO₂ and an S-value of about 27%.

[0037] To 5000 g of this acid silica sol was added 239 g of a 34%choline hydroxide solution under agitation for about 20 seconds,resulting in an amine stabilized aqueous silica sol with a molar ratioof SiO₂ to N of 11:1. In order to reduce the smell from the cholinehydroxide, 5.0 g of limonene was added. The final silica-based sol had apH of 10.8, SiO₂ content of 8.73% by weight, S-value of 20% andcontained silica particles with a specific surface area of 820 m²/gSiO₂.

EXAMPLE 2

[0038] Sodium waterglass with a molar ratio of SiO₂ to Na₂O of 3.4 wasdiluted to about 6% by weight SiO₂ and treated with a cationicion-exchange resin saturated with hydrogen ions. The obtained acidsilica sol, or polysilicic acid, had a pH of 2.4, SiO₂ content of 5.7%by weight and contained silica particles with a specific surface area of1350 m²/g SiO₂ and an S-value of about 32%.

[0039] To 2000 g of this polysilicic acid was added 120 g of a 34%choline hydroxide solution under agitation for 2 seconds, resulting inan amine stabilized aqueous silica-based sol which had a pH of 10.4,SiO₂ content of 5.4% by weight, molar ratio SiO₂ to N of 5.5:1, S-valueof 28% and contained silica particles with a specific surface area of1330 m²/g SiO₂.

EXAMPLE 3

[0040] A sodium hydroxide stabilized silica sol was deionized in thesame manner as in Example 1 resulting in an acid silica sol which had apH of 2.4, SiO₂ content of 9.15% and contained silica particles with aspecific surface area of 850 m²/g SiO₂.

[0041] To 2000 g of this acid silica sol was added 90 g of a 25%solution of tetramethylammonium hydroxide under agitation for 2 seconds.The obtained silica-based sol had a pH of 10.4, SiO₂ content of 8.75%,molar ratio of SiO₂ to N of 12:1, S-value of 19.5% and contained silicaparticles with a specific surface area of 850 m²/g SiO₂.

EXAMPLE 4

[0042] A dilute sodium waterglass solution was ion-exchanged in the samemanner as in Example 2. The resulting polysilicic acid had a pH of 2.4,SiO₂ content of 5.8% by weight and contained silica particles with aspecific surface area of 1365 m²/g SiO₂.

[0043] To 10000 g of this polysilicic acid was added 552 g of a 25%solution of tetramethylammonium hydroxide under agitation for 20seconds. The resulting amine stabilized alkaline silica sol, which hadan SiO₂ content of 5.4% and a molar ratio SiO₂ to N of 6.7:1, wasconcentrated by ultrafiltration to a stable silica-based sol which had apH of 10.5, SiO₂ content of 13.4% by weight, S-value of 27% andcontained silica particles with a specific surface area of 1140 m²/gSiO₂.

EXAMPLE 5

[0044] A sodium hydroxide stabilized silica sol was deionized in thesame manner as in Example 1 resulting in an acid silica sol had a pH of2.5, SiO₂ content of 8.7% by weight and contained silica particles witha specific surface area of 860 m²/g SiO₂.

[0045] To 1750 g of this acid silica sol was added 70 g of a 35%solution of tetraethylammonium hydroxide under agitation for 1 second.The obtained amine stabilized alkaline silica-based sol had a pH of10.8, SiO₂ content of 8.4% by weight, molar ratio SiO₂ to N of 15:1,S-value of 21% and contained silica particles with a specific surfacearea of 930 m²/g SiO₂.

EXAMPLE 6

[0046] A sodium hydroxide stabilized silica sol was deionized in thesame manner as in Example 1 resulting in an acid silica sol with a pH of2.4, SiO₂ content of 8.9% by weight and silica particles with a specificsurface area of 820 m²/g SiO₂.

[0047] To 2000 g of this acid silica sol was added 214 g of a 20%solution of tetrapropylammonium hydroxide under agitation for 15seconds. The obtained aqueous silica-based sol had a pH of 10.7, SiO₂content of 8.1% by weight, molar ratio SiO₂ to N of 14:1, S-value of 24%and contained silica particles with a specific surface area of 820 m²/gSiO₂.

EXAMPLE 7

[0048] A sodium hydroxide stabilized silica sol containing silicaparticles with a specific surface area of around 800 m²/g SiO₂ wasdeionized in the same manner as in Example 1 resulting in an acid silicasol with a pH of 2.6, SiO₂ content of 9.3% by weight and containedsilica particles with a specific surface area of 795 m²/g SiO₂,

[0049] To 2000 g of this acid silica sol was added 192.4 g oftriethanolamine under agitation for 10 seconds. The obtainedsilica-based sol had a pH of 9.0, SiO₂ content of 8.5%, molar ratio SiO₂to N of 2.4:1, S-value of 15% and contained silica particles with aspecific surface area of 795 m²/g SiO₂.

EXAMPLE 8

[0050] To 2000 g of the acid silica sol according to Example 7 was added30.1 g of triethylamine under agitation for 10 seconds. The obtainedsilica-based sol had a pH of 10.2, SiO₂ content of 9.15%, molar ratioSiO₂ to N of 10.4:1, S-value of 25% and contained silica particles witha specific surface area of 800 m²/g SiO₂.

EXAMPLE 9

[0051] A sodium hydroxide stabilized silica sol was deionized in thesame manner as in Example 1 resulting in an acid silica sol with a pH of2.8, SiO₂ content of 9.3% by weight and contained silica particles witha specific surface area of 860 m²/g SiO₂.

[0052] To 2000 g of this acid silica sol was added 68.1 g ofN,N-dimethyletanolamine under agitation for 5 seconds. The obtainedsilica-based sol had a pH of 10.1, SiO₂ content of 9.0%, molar ratioSiO₂ to N of 4:1, S-value of 26% and contained silica particles with aspecific surface area of 860 m²/g SiO₂.

EXAMPLE 10

[0053] A sodium hydroxide stabilized silica sol with specific surfacearea around 800 m²/g was deionized in the same manner as in Example 1resulting in an acid silica sol which had a pH of 2.6, SiO₂ content of9.1% by weight and contained silica particles with a specific surfacearea of 880 m²/g SiO₂,

[0054] To 2000 g of this acid silica sol was added 103 g ofdiethanolamine under agitation for 2 seconds. The obtained aminestabilized alkaline silica sol had pH of 10.1, SiO₂ content of 8.65%,molar ratio SiO₂ to N of 3:1, S-value of 22% and contained silicaparticles with a specific surface area of 875 m²/g SiO₂.

EXAMPLE 11

[0055] To 2000 g of the acid silica sol according to Example 10 wasadded 40.4 g of diethylamine under agitation for 2 seconds. The obtainedsilica-based sol had a pH of 11.4, SiO₂ content of 8.92%, molar ratioSiO₂ to N of 6.5:1, S-value of 22% and contained silica particles with aspecific surface area of 880 m²/g SiO₂.

EXAMPLE 12

[0056] To 2000 g of the acid silica sol according to Example 10 wasadded 32.4 g of diisopropylamine under agitation for 2 seconds. Theobtained silica-based sol had a pH of 11.0, SiO₂ content of 8.95%, molarratio SiO₂ to N of 9.5:1, S-value of 25% and contained silica particleswith a specific surface area of 885 m²/g SiO₂.

EXAMPLE 13

[0057] To 2000 g of the acid silica sol according to Example 10 wasadded 32.5 g of pyrrolidine under agitation for 2 seconds. The obtainedsilica-based sol had a pH of 11.1, SiO₂ content of 8.95%, molar ratioSiO₂ to N of 6.6:1, S-value of 25% and contained silica particles with aspecific surface area of 880 m²/g SiO₂.

EXAMPLE 14

[0058] To 2000 g of another deionized silica sol, which had pH of 2.8,SiO₂ content of 9.3% and contained silica particles with a specificsurface area of 860 m²/g SiO₂, was added 35.5 g of dipropylamine underagitation for 2 seconds. The obtained silica-based sol had a pH of 10.6,SiO₂ content of 9.10%, molar ratio SiO₂ to N of 8.8:1, S-value of 30%and contained silica particles with a specific surface area of 855 m²/gSiO₂.

EXAMPLE 15

[0059] To 2000 g of acid silica sol according to Example 10 was added33.7 g of ethanolamine under agitation for 2 seconds. The resultingsilica-based sol had a pH of 10.1, SiO₂ content of 8.95%, molar ratioSiO₂ to N of 5.5:1, S-value of 24% and contained silica particles with aspecific surface area of 870 m²/g SiO₂.

EXAMPLE 16

[0060] To 2000 g of the acid silica sol according to Example 10 wasadded 30 g of cyclohexylamine under agitation for 2 seconds. Theresulting silica-based sol had pH of 10.4, SiO₂ content of 9.0%, molarratio SiO₂ to N of 10:1, S-value of 24% and contained silica particleswith a specific surface area of 880 m²/g SiO₂.

EXAMPLE 17

[0061] To 2000 g of another deionized silica sol, which had pH of 2.8,SiO₂ content of 9.3% and contained silica particles with a specificsurface area of 860 m²/g SiO₂, was added 59.1 g of 2-methoxyethylamineunder agitation for 2 seconds. The obtained silica-based sol had a pH of10.2, SiO₂ content of 9.0%, molar ratio SiO₂ to N of 3.9:1, S-value of28% and contained silica particles with a specific surface area of 850m²/g SiO₂.

EXAMPLE 18

[0062] To 1500 g of deionized silica sol, which had pH of 2.8, SiO₂content of 9.3% and contained silica particles with a specific surfacearea of 860 m²/g SiO₂, was added 66.1 g of aminoethylethanolamine underagitation for 5 seconds. The obtained silica-based sol had a pH of 10.5,SiO₂ content of 9.0%, molar ratio SiO₂ to amine of 3.6:1, S-value of 26%and contained silica particles with a specific surface area of 875 m2/gSiO₂.

EXAMPLE 19

[0063] In the following tests, drainage and retention performance ofsilica-based sols according to Examples 1 to 18 were tested. Drainageperformance was evaluated by means of a Dynamic Drainage Analyser (DDA),available from Akribi, Sweden, which measures the time for draining aset volume of stock through a wire when removing a plug and applying avacuum to that side of the wire opposite to the side on which the stockis present. Retention performance was evaluated by means of anephelometer by measuring the turbidity of the filtrate, the whitewater, obtained by draining the stock.

[0064] The stock used was based on a standard fine paper furnishconsisting of 60% bleached birch sulfate and 40% bleached pine sulfate.30% calcium carbonate was added to the stock as filler and 0.3 g/l ofNa₂SO₄.10 H₂O was added to increase conductivity. Stock pH was 8.4,conductivity 0.46 mS/cm and consistency 0.29%. In the tests, thesilica-based sols were tested in conjunction with a cationic polymerbeing a cationic starch having a degree of substitution of about 0.042.The starch was added in an amount of 12 kg/tonne, calculated as drystarch on dry stock system, and the silica based sols were added inamounts of 0.25, 0.5 and 1.0 kg/tonne calculated as dry silica on drystock system.

[0065] The silica-based sols according to the invention were testedagainst two silica-based sols, Ref. 1 and Ref. 2, used for comparativepurposes. Ref. 1 is a silica-based of the type disclosed in U.S. Pat.No. 5,368,833 which had an S-value of about 25%, SiO₂ content of 8%,specific surface area of 72 m²/g aqueous sol and contained silicaparticles with a specific surface area of about 900 m²/g SiO₂ which weresurface-modified with aluminium to a degree of 5%. Ref. 2 is a silicasol with an S-value of 36%, SiO₂ content of 10.0%, molar ratio SiO₂ toNa₂O of 10:1, specific surface area of 88 m²/g aqueous sol andcontaining silica particles with a specific surface area of 880 m²/gSiO₂.

[0066] The stock was stirred in a baffled jar at a speed of 1500 rpmthroughout the test and chemical additions to the stock were conductedas follows:

[0067] i) adding cationic polymer followed by stirring for 30 seconds,

[0068] ii) adding silica-based particles followed by stirring for 15seconds,

[0069] iii) draining the stock while automatically recording thedrainage time.

[0070] Table I shows the results obtained when using varying dosages(kg/tonne, calculated as SiO₂ and based on dry stock system) ofsilica-based sol. Without addition of chemicals, the stock showed adrainage time of 20 seconds and a turbidity of 490 NTU. With addition ofcationic starch only, 12 kg/tonne, calculated as dry starch on dry stocksystem, the stock showed a drainage time of 15 seconds and a turbidityof 70 NTU. TABLE I Drainage time (sec)/Turbidity (NTU) at SiO₂ dosage ofSilica-based sol 0.25 kg/t 0.5 kg/t 1.0 kg/t Ref. 1 12.20/43 10.40/40 8.76/37 Ref. 2 11.60/45 9.83/44 8.28/37 Example 1  9.11/38 7.19/305.74/28 Example 2  8.65/38 6.79/35 5.76/— Example 3  9.34/40 7.30/346.30/28 Example 4  8.82/39 6.97/36 5.86/31 Example 5 —/— 7.74/37 —/—Example 6 —/— 8.98/38 —/— Example 7  10.3/42 8.77/37 6.66/33 Example 8 10.3/42 8.31/36 7.02/33 Example 9  9.90/— 8.80/— 7.90/— Example 1010.00/— 8.21/— 7.07/— Example 11 10.00/— 8.04/— 7.28/— Example 12 9.87/— 7.97/— 6.85/— Example 13  9.60/— 7.85/— 6.30/— Example 1410.70/— 8.80/— 7.80/— Example 15 10.70/— 8.80/— 7.51/— Example 1610.30/— 8.13/— 6.75/— Example 17 10.50/— 8.80/— 7.70/— Example 1810.60/— 9.20/— 8.20/—

EXAMPLE 20

[0071] In the following tests, drainage and retention performance of thesilica-based sol according to Example 3 was further evaluated. Theprocedure according to Example 19 was followed except that a differentstock and different cationic polymers were used.

[0072] The furnish was based on 70% cellulosic fibres and 30% clayfiller. The fibres consisted of about 70% bleached thermomechanicalpulp, 10% stoneground pulp, 10% bleached birch sulphate and 10% bleachedpine sulphate pulp. The pulp and filler was dispersed in water to aconsistency of 1.5 g/l. In the water was included 25 g/l of bleach waterfrom a bleaching plant containing dissolved organic disturbingsubstances and calcium chloride (CaCl₂.10 H₂O) in an amount to give aconductivity of 5 mS/cm.

[0073] The silica-based sols were used in combination with a highlycationic low molecular weight polyamine, which was added in an amount of0.5 kg/tonne, calculated dry polymer on dry stock system, and a cationicpolyacrylamide, which was added in an amount of 1.0 kg/tonne, calculateddry polymer on dry stock system. The polyamine was added to the stocksystem followed by stirring for 15 seconds and then the cationicpolyacrylamide and silica-based sol were added according to theprocedure of Example 19. The silica based sols were added in amounts of0.25, 0.5 and 1.0 kg/tonne calculated as dry silica on dry stock system.

[0074] Table II shows the results obtained when using varying dosages(kg/tonne, calculated as SiO₂ and based on dry stock system) ofsilica-based sol. Without addition of chemicals, the stock showed adrainage time of 22 seconds and a turbidity of 100 NTU. With addition ofsolely 1 kg/tonne cationic polyacrylamide, calculated as dry polymer ondry stock system, the stock showed a drainage time of 16 seconds and aturbidity of 55 NTU. With addition of 0.5 kg/tonne cationic polyamineand 1 kg/tonne cationic polyacrylamide, calculated as dry polymers ondry stock system, the stock showed a drainage time of 11 seconds and aturbidity of 50 NTU. TABLE II Drainage time (sec)/Turbidity (NTU) atSiO₂ dosage of Silica-based sol 0.25 kg/t 0.5 kg/t 1.0 kg/t Ref. 112.20/48 11.00/47 9.90/45 Ref. 2 12.30/47 10.70/43 9.18/41 Example 310.10/40  8.08/39 6.27/40

EXAMPLE 21

[0075] Sodium waterglass with a ratio SiO₂ to Na₂O of 3.4:1 was dilutedto around 6% SiO₂ and treated with a cationic ion-exchange resinsaturated with hydrogen ions. The obtained polysilicic acid had a pH of2.5, SiO₂ content of 5.6%, and contained silica particles with aspecific surface area of 1300 m²/g SiO₂.

[0076] To 5000 g of this polysilicic acid was added 353.5 g of a 34%choline hydroxide solution under agitation for 5 seconds, resulting inan amine stabilized alkaline silica-based sol with a pH of 10.8, SiO₂content of 5.26% and molar ratio SiO₂ to N of 4.6. This sol wasconcentrated by vacuum-evaporation to a stable silica-based sol whichhad an SiO₂ content of 13.9% by weight, S-value about 30% and a specificsurface area of 169 m²/g aqueous sol (measured after 40 days) andcontained silica particles with a specific surface area of 1215 m²/gSiO₂ (measured after 40 days). The viscosity was essentially constantduring these 40 days; initially 11.8 cP and 11.0 cP after 40 days.

EXAMPLE 22

[0077] To 5000 g of the polysilicic acid according to Example 21 wasadded 347.2 g of a 35% tetraethylammonium hydroxide solution underagitation for 5 seconds. The resulting amine stabilized alkaline silicasol had a pH of 10.8, SiO₂ content of 5.26% and molar ratio SiO₂ to N of5.7:1. This sol was concentrated by vacuum-evaporation to a stablesilica-based sol which had an SiO₂ content of 20.0%, viscosity of 9.9 cPand specific surface area of 250 m²/g aqueous sol, and containedsilica-based particles with a specific surface area of 1250 m²/g SiO₂.After storage for 40 days, the sol showed a viscosity of 8.2 cP, S-valueof 43% and specific surface areas of 239 m²/g aqueous sol and 1195 m²/gSiO₂.

EXAMPLE 23

[0078] To 5000 g polysilicic acid having a SiO₂ content of 5.1% preparedin a manner similar to Example 21 was added 114 g of dipropylamine underagitation for 5 seconds. The obtained amine stabilized alkalinesilica-based sol had pH of 10.8, SiO₂ content of 5.0% and molar ratioSiO₂ to N of 3.8:1. This sol was concentrated by ultrafiltration to astable silica-based sol which had an SiO₂ content of 14.8%, specificsurface area of 196 m²/g aqueous sol and contained silica particles witha specific surface area of 1320 m²/g SiO₂.

EXAMPLE 24

[0079] To 5000 g polysilicic acid having a SiO₂ content of 5.5% preparedin a manner similar to Example 21 was added 229.8 gaminoethylethanolamine under agitation for 5 seconds, resulting in anamine stabilized alkaline silica sol with a pH of 10.3, SiO₂ content of5.2% and molar ratio SiO₂ to N of 2:1. This sol was concentrated byvacuum-evaporation to a stable silica-based sol with an SiO₂ content of13.6% and specific surface areas of 170 m²/g aqueous sol and 1255 m²/gSiO₂.

EXAMPLE 25

[0080] A sodium hydroxide stabilized silica sol having a SiO₂ content of15% by weight, S-value of about 50% and containing silica particles witha specific surface area of 500 m²/g SiO₂ was deionized in the samemanner as in example 1 resulting in an acid silica sol exhibiting a pHof 2.9, SiO₂ content of 14.8% by weight and specific surface area of 490m²/g SiO₂.

[0081] To 4000 g of this acid sol was added 414.5 g of a 35% solution oftetraethylammonium hydroxide under agitation for 5 seconds, resulting inan amine stabilized alkaline silica-based sol with a pH of 12.1, SiO₂content of 13.4% and molar ratio SiO₂ to N of 10:1. This sol wasconcentrated by vacuum-evaporation to a stable silica-based solexhibiting a SiO₂ content of 40%, specific surface areas of 224 m²/gaqueous sol and 560 m²/g SiO₂.

EXAMPLE 26

[0082] The drainage (dewatering) and retention performance of thesilica-based sols according to Examples 21-24 was investigated in amanner similar Example 19. The results are set forth in Table III. TABLEIII Drainage time (sec)/Turbidity (NTU) at SiO₂ dosage of Silica-basedsol 0.25 kg/t 0.5 kg/t 1.0 kg/t Ref. 1 12.1/49 10.2/43  9.1/43 Ref. 211.8/50 10.2/50  9.0/42 Example 21  9.3/36 7.4/34 6.9/34 Example 2210.3/45 9.3/42 8.8/41 Example 23 10.8/44 8.8/42 8.1/37 Example 2410.0/44 8.7/40 7.9/38

EXAMPLE 27

[0083] To 1024 g of polysilicic acid having a pH of 2.7 and SiO₂ contentof 5.84% by weight, prepared in a manner similar to Example 21, wasadded 37.1 g of a 75% by weight solution of choline chloride underagitation resulting in a molar ratio of SiO₂ to N of 5.0. To thismixture was added 99.6 g of 3M NaOH under agitation. The obtainedsilica-based sol had a pH of 11.0, SiO₂ content of 5.1% by weight, andcontained silica particles with a specific surface area of 1010 m²/gSiO₂.

EXAMPLE 28

[0084] To 1068 g of polysilicic acid according to Example 27 was added amixture of 39 g of a 75% solution of choline chloride and 99.6 g of 3MNaOH under agitation. The obtained silica-based sol had a pH of 11.0,molar ratio SiO₂ to N of 5.0, SiO₂ content of 5.2% by weight andcontained silica-based particles with a specific surface area of 1175m²/g SiO₂.

EXAMPLE 29

[0085] To 50 g of polysilicic acid according to Example 27 was added 0.9g of a 75% solution of choline chloride under agitation resulting in amolar ratio SiO₂ to N of 10.0. The obtained mixture was added to 9.5 gof 3M NaOH under agitation. The obtained silica-based sol had a pH of10.0.

EXAMPLE 30

[0086] 50 g of polysilicic acid according to Example 27 was added to amixture of 0.9 g of a 75% by weight solution of choline chloride and 9.5g of 3M NaOH under agitation. The obtained silica-based sol had a pH of10.1, molar ratio SiO₂ to N of 10.0 and SiO₂ content of 4.8% by weight.

EXAMPLE 31

[0087] To 50 g polysilicic acid according to Example 27 was added 0.9 gof a 75% by weight solution of choline chloride under agitationresulting in a molar ratio SiO₂ to N of 10.0. To this mixture was added20.0 g of a water glass solution containing 9.2% by weight SiO₂ underagitation resulting in a silica-based sol with a pH of 10.1 and SiO₂content of 6.6% by weight.

EXAMPLE 32

[0088] The drainage (dewatering) and retention performance of thesilica-based sols according to Examples 27-31 was investigated in amanner similar Example 19 except that calcium chloride was added to thestock to increase the conductivity to 2.0 mS/cm and that the cationicstarch was added in an amount of 10 kg/tonne, calculated as dry starchon dry stock system. The results are set forth in Table IV. TABLE IVDrainage time (sec)/Turbidity (NTU) at SiO₂ dosage of Silica-based sol0.5 kg/t 1.0 kg/t Ref. 1 25.5/120  21.4/104 Example 27 23.4/116 16.1/83Example 28 23.1/115 14.5/91 Example 29 22.3/102 16.2/85 Example 3020.9/98  14.4/78 Example 31 23.0/110 17.3/87

EXAMPLE 33

[0089] An alkali stabilized silica sol was treated with a strong cationexchange resin in acid form and after the treatment the silica sol had apH of 2.6, SiO₂ content of 16.2% by weight and specific surface area of500 m²/g. To 2530 g of this silica sol was added 294 g of tetraethanolammonium hydroxide under rapid agitation for about 5 seconds resultingin an amine stabilized silica-based sol with a pH of 11.1, SiO₂ contentof 14.5% by weight and molar ratio SiO₂ to N of 4.9:1. The viscosity was4.6 cP and specific surface area 500 m²/g. The obtained product was usedfor drainage performance testing.

[0090] It was possible to concentrate this product by evaporation ofwater to a SiO₂ content of 38.8% by weight. The concentrated product hadessentially the same specific surface area and could be stored for 6weeks without gelling. The concentrated product could easily be dilutedand showed the same good performance in drainage as the non-concentratedproduct.

EXAMPLE 34

[0091] In the following tests, drainage performance of thenon-concentrated silica-based sol according to Example 33 was tested inthe same manner as in Example 19. The stock was the same as in Example19, but calcium chloride was added in an amount to increase theconductivity to 2.0 mS/cm. Stock pH was 8.4 and consistency 0.3% byweight. Cationic starch dosage was 12 kg/t, calculated as dry starch ondry stock. The silica-based sol according to Example 33 was testedagainst Ref. 3, which is a silica sol with an S-value of about 50, SiO₂content of 15.0% by weight, specific surface area of 75 m²/g aqueous soland containing silica particles with a specific surface area of about500 m²/g of SiO₂. The results are set forth in Table V. TABLE V Drainagetime (sec)/Turbidity (NTU) at SiO₂ dosage of Silica-based sol 0.25 kg/t0.5 kg/t 1.0 kg/t Ref. 3 28.0 23.9 17.8 Example 33 26.2 22.6 17.4

EXAMPLE 35

[0092] To 3000 g of a polysilicic acid having a pH of 2.6, SiO₂ contentof 5.7% by weight and specific surface area of 1270 m²/g was added 200 gof tetraethanol ammonium hydroxide under rapid agitation for about 5seconds resulting in an amine stabilized silica-based sol with a pH of9.9, SiO₂ content of 5.4% by weight and molar ratio SiO₂ to N of 3:1.The viscosity was 3.2 cP and specific surface area 1160 m²/g. Theobtained product was used for drainage performance testing.

[0093] It was possible to concentrate this product by evaporation ofwater to a SiO₂ content of 14.5% by weight. The concentrated product wasstorage stable for more than 2 months and showed the same good drainageperformance as the non-concentrated product.

EXAMPLE 36

[0094] In the following tests, drainage performance of thenon-concentrated silica-based sol according to Example 35 was tested inthe same manner as in Example 19. The stock was the same as in Example19, but calcium chloride was added in an amount to increase theconductivity to 2.0 mS/cm. Stock pH was 8.4 and consistency 0.3% byweight. Cationic starch dosage was 12 kg/t, calculated as dry starch ondry stock. The silica-based sol according to Example 35 was testedagainst Ref. 2. The results are set forth in Table VI. TABLE V Drainagetime (sec)/Turbidity (NTU) at SiO₂ dosage of Silica-based sol 0.25 kg/t0.5 kg/t 1.0 kg/t Ref. 2 25.4 21.0 14.9 Example 35 22.7 17.4 13.8

1. Aqueous silica-based sol comprising a nitrogen-containing organiccompound and silica-based particles with a specific surface area of atleast 300 square meters per gram of silica and having an S-value withinthe range of from 10 to 60%.
 2. The aqueous silica-based sol of claim 1, wherein it contains silica-based particles with a specific surfacearea of from 550 to 1700 square meters per gram of silica.
 3. Theaqueous silica-based sol of claim 1 , wherein the nitrogen-containingorganic compound has a molecular weight below 1,000.
 4. The aqueoussilica-based sol of claim 1 , wherein the S-value is within the range offrom 15 to 40%.
 5. The aqueous silica-based sol of claim 1 , wherein ithas a specific surface area in the range of from 150 to 250 squaremeters per gram of aqueous sol.
 6. The aqueous silica-based sol of claim1 , wherein it has a silica content in the range of from 10 to 60% byweight.
 7. The aqueous silica-based sol of claim 1 , wherein it has a pHof from 8 to
 13. 8. The aqueous silica-based sol of claim 1 , whereinthe nitrogen-containing organic compound is an amine containing from 2to 12 carbon atoms.
 9. The aqueous silica-based sol of claim 1 , whereinthe nitrogen-containing organic compound is a quaternary amine.
 10. Theaqueous silica-based sol of claim 1 , wherein the nitrogen-containingorganic compound is selected from the group consisting of propylamine,butylamine, cyclohexylamine, ethanolamine, 2-methoxyethylamine,diethylamine, dipropylamine diisopropylamine, diethanolamine,pyrrolidine, triethylamine, triethanolamine, N,N-dimethylethanolamine,tetraethanol ammonium hydroxide, tetraethanol ammonium chloride,methyltriethanolammonium hydroxide, methyltriethanolammonium chloride,dimethyldiethanolammonium hydroxide, dimethyldiethanolammonium chloride,choline hydroxide, choline chloride, dimethylcocobenzylammoniumhydroxide, dimethylcocobenzylammonium chloride, tetramethylammoniumhydroxide, tetramethylammonium chloride, tetraethylammonium hydroxide,tetraethylammonium chloride, tetrapropylammonium hydroxide,tetrapropylammonium chloride, diethyldimethylammonium hydroxide,diethyldimethylammonium chloride, triethylmethylammonium hydroxide,triethylmethylammonium chloride, aminoethylethanolamine, piperazine andderivatives thereof, and mixtures thereof.
 11. The aqueous silica-basedsol of claim 10 , wherein the nitrogen-containing organic compound isselected from the group consisting of triethanolamine, diethanolamine,dipropylamine, aminoethylethanolamine, 2-methoxyethylamine,N,N-dimethylethanolamine, choline hydroxide, choline chloride,tetramethylammonium hydroxide, tetraethylammonium hydroxide, andtetraethanol ammonium hydroxide.
 12. The aqueous silica-based sol ofclaim 1 , wherein the nitrogen-containing organic compound contains atleast one oxygen atom.
 13. Aqueous silica-based sol comprising anitrogen-containing organic compound and silica-based particles with aspecific surface area of at least 700 square meters per gram of silica.14. The aqueous silica-based sol of claim 13 , wherein thenitrogen-containing organic compound has a molecular weight below 1,000.15. The aqueous silica-based sol of claim 13 , wherein it has an S-valuewithin the range of from 15 to 40%.
 16. The aqueous silica-based sol ofclaim 13 , wherein it has a specific surface area in the range of from150 to 250 square meters per gram of aqueous sol.
 17. The aqueoussilica-based sol of claim 13 , wherein it has a silica content in therange of from 10 to 60% by weight.
 18. The aqueous silica-based sol ofclaim 13 , wherein it has a pH of from 8 to
 13. 19. The aqueoussilica-based sol of claim 13 , wherein the nitrogen-containing organiccompound is an amine containing from 2 to 12 carbon atoms.
 20. Theaqueous silica-based sol of claim 13 , wherein the nitrogen-containingorganic compound is a quaternary amine.
 21. The aqueous silica-based solof claim 13 , wherein the nitrogen-containing organic compound isselected from the group consisting of propylamine, butylamine,cyclohexylamine, ethanolamine, 2-methoxyethylamine, diethylamine,dipropylamine diisopropylamine, diethanolamine, pyrrolidine,triethylamine, triethanolamine, N,N-dimethylethanolamine, tetraethanolammonium hydroxide, tetraethanol ammonium chloride,methyltriethanolammonium hydroxide, methyltriethanolammonium chloride,dimethyldiethanolammonium hydroxide, dimethyldiethanolammonium chloride,choline hydroxide, choline chloride, dimethylcocobenzylammoniumhydroxide, dimethylcocobenzylammonium chloride, tetramethylammoniumhydroxide, tetramethylammonium chloride, tetraethylammonium hydroxide,tetraethylammonium chloride, tetrapropylammonium hydroxide,tetrapropylammonium chloride, diethyldimethylammonium hydroxide,diethyldimethylammonium chloride, triethylmethylammonium hydroxide,triethylmethylammonium chloride, aminoethylethanolamine, piperazine andderivatives thereof, and mixtures thereof.
 22. The aqueous silica-basedsol of claim 21 , wherein the nitrogen-containing organic compound isselected from the group consisting of triethanolamine, diethanolamine,dipropylamine, aminoethylethanolamine, 2-methoxyethylamine,N,N-dimethylethanolamine, choline hydroxide, choline chloride,tetramethylammonium hydroxide, tetraethylammonium hydroxide, andtetraethanol ammonium hydroxide.
 23. The aqueous silica-based sol ofclaim 13 , wherein the nitrogen-containing organic compound contains atleast one oxygen atom.
 24. Aqueous silica-based sol having a specificsurface area of at least 100 square meters per gram of aqueous sol andcomprising a nitrogen-containing organic compound and silica-basedparticles with a specific surface area of at least 300 square meters pergram of silica.
 25. The aqueous silica-based sol of claim 24 , whereinthe silica-based particles have a specific surface area of from 550 to1700 square meters per gram of silica.
 26. The aqueous silica-based solof claim 24 , wherein the nitrogen-containing organic compound has amolecular weight below 1,000.
 27. The aqueous silica-based sol of claim24 , wherein it has an S-value within the range of from 15 to 40%. 28.The aqueous silica-based sol of claim 24 , wherein it has a specificsurface area in the range of from 150 to 250 square meters per gram ofaqueous sol.
 29. The aqueous silica-based sol of claim 24 , wherein ithas a silica content in the range of from 10 to 60% by weight.
 30. Theaqueous silica-based sol of claim 24 , wherein it has a pH of from 8 to13.
 31. The aqueous silica-based sol of claim 24 , wherein thenitrogen-containing organic compound is an amine containing from 2 to 12carbon atoms.
 32. The aqueous silica-based sol of claim 24 , wherein thenitrogen-containing organic compound is a quaternary amine.
 33. Theaqueous silica-based sol of claim 24 , wherein the nitrogen-containingorganic compound is selected from the group consisting of propylamine,butylamine, cyclohexylamine, ethanolamine, 2-methoxyethylamine,diethylamine, dipropylamine diisopropylamine, diethanolamine,pyrrolidine, triethylamine, triethanolamine, N,N-dimethylethanolamine,tetraethanol ammonium hydroxide, tetraethanol ammonium chloride,methyltriethanolammonium hydroxide, methyltriethanolammonium chloride,dimethyldiethanolammonium hydroxide, dimethyldiethanolammonium chloride,choline hydroxide, choline chloride, dimethylcocobenzylammoniumhydroxide, dimethylcocobenzylammonium chloride, tetramethylammoniumhydroxide, tetramethylammonium chloride, tetraethylammonium hydroxide,tetraethylammonium chloride, tetrapropylammonium hydroxide,tetrapropylammonium chloride, diethyldimethylammonium hydroxide,diethyldimethylammonium chloride, triethylmethylammonium hydroxide,triethylmethylammonium chloride, aminoethylethanolamine, piperazine andderivatives thereof, and mixtures thereof.
 34. The aqueous silica-basedsol of claim 33 , wherein the nitrogen-containing organic compound isselected from the group consisting of triethanolamine, diethanolamine,dipropylamine, aminoethylethanolamine, 2-methoxyethylamine,N,N-dimethylethanolamine, choline hydroxide, choline chloride,tetramethylammonium hydroxide, tetraethylammonium hydroxide, andtetraethanol ammonium hydroxide.
 35. The aqueous silica-based sol ofclaim 24 , wherein the nitrogen-containing organic compound contains atleast one oxygen atom.
 36. Process for the production of an aqueoussilica-based sol which comprises incorporating a nitrogen-containingorganic compound in a silica-based sol containing silica-based particleswith a specific surface area of at least 300 square meters per gram ofsilica and having an S-value in the range of from 10 to 60%.
 37. Theprocess of claim 36 , wherein the nitrogen-containing organic compoundhas a molecular weight below 1,000.
 38. The process of claim 36 ,wherein the aqueous silica-based sol contains silica-based particleswith a specific surface area of from 550 to 1700 square meters per gramof silica.
 39. The process of claim 36 , wherein the aqueoussilica-based sol obtained is concentrated to a specific surface area inthe range of from 150 to 250 square meters per gram of aqueous sol. 40.The process of claim 36 , wherein the nitrogen-containing organiccompound is an amine containing from 2 to 12 carbon atoms.
 41. Theprocess of claim 36 , wherein the nitrogen-containing organic compoundcontains at least one oxygen atom.
 42. Process for the production of anaqueous silica-based sol which comprises incorporating anitrogen-containing organic compound in a silica-based sol containingsilica-based particles with a specific surface area of at least 700square meters per gram of silica.
 43. The process of claim 42 , whereinthe nitrogen-containing organic compound has a molecular weight below1,000.
 44. The process of claim 42 , wherein the aqueous silica-basedsol contains silica-based particles with a specific surface area of from550 to 1700 square meters per gram of silica.
 45. The process of claim42 , wherein the aqueous silica-based sol obtained is concentrated to aspecific surface area in the range of from 150 to 250 square meters pergram of aqueous sol.
 46. The process of claim 42 , wherein thenitrogen-containing organic compound is an amine containing from 2 to 12carbon atoms.
 47. The process of claim 42 , wherein thenitrogen-containing organic compound contains at least one oxygen atom.48. Process for the production of an aqueous silica-based sol whichcomprises incorporating a nitrogen-containing organic compound in asilica-based sol containing silica-based particles with a specificsurface area of at least 300 square meters per gram of silica andconcentrating the aqueous silica-based sol obtained to a specificsurface area of at least 100 square meters per gram of aqueous sol. 49.The process of claim 48 , wherein the nitrogen-containing organiccompound has a molecular weight below 1,000.
 50. The process of claim 48, wherein the aqueous silica-based sol contains silica-based particleswith a specific surface area of from 550 to 1700 square meters per gramof silica.
 51. The process of claim 48 , wherein the aqueoussilica-based sol obtained is concentrated to a specific surface area inthe range of from 150 to 250 square meters per gram of aqueous sol. 52.The process of claim 48 , wherein the nitrogen-containing organiccompound is an amine containing from 2 to 12 carbon atoms.
 53. Theprocess of claim 48 , wherein the nitrogen-containing organic compoundcontains at least one oxygen atom.
 54. Process for the production ofpaper from an aqueous suspension containing cellulosic fibres, andoptional fillers, which comprises adding to the suspension an aqueoussilica-based sol comprising an organic nitrogen-containing compound andat least one charged organic polymer, forming and draining thesuspension on a wire.
 55. The process of claim 54 , wherein the solcontains silica-based particles with a specific surface area of at least300 square meters per gram of silica.
 56. The process of claim 54 ,wherein the charged organic polymer comprises a cationic starch and/orcationic polyacrylamide.